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Irish Zn/Pb Deposits: Structure and Fluid Flow |
Department of Geology and Applied Geology, University of Glasgow, Glasgow, G12 8QQ, UK
Department of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK
Much is known about the Carboniferous-hosted, hydrothermal, Zn-Pb deposits of central Ireland, including temperatures, geochemistries and fluid properties at the mineralized sites. However, this knowledge is insufficient to force a unique genetic interpretation of the palaeohydrogeological system responsible for mineralization. Two hypothetical flow systems survive previous mass-balance testing: a deep, topographically driven, regional flow, and deep, convective flow systems local to each deposit. In order to develop models of these flow systems a 4D, Early Carboniferous restoration of basin development in central Ireland has been created. The structural style is one of tilted fault blocks of the pre-Carboniferous ‘basement’; the Carboniferous sedimentary rocks are deposited in the accommodation space created by movement of the basement. Fluid and heat flow simulations based on the geological restorations show that most of the rising, hot flows are focused on faults. The 4D restorations suggest that such faults are at active, syn-depositional, fault-block corners. Buoyancy driven convective flow systems, operating within both the fractured basement and its sedimentary cover, are a viable mineralization system that agrees with the geological interpretation. Topographically driven flow systems developed from the geological interpretation do not reproduce the important characteristics of hydrothermal flow as deduced from evidence at the deposit sites.
